Embodiments of this invention relate to tunable chromatic dispersion compensation devices. More particularly, embodiments relate to tunable chromatic dispersion compensator using dual fibers having a given spacing between their cores.
A single narrow pulse of light may consist of many wavelengths in a given passband entering a fiber optic transmission system. During the travels through the fiber optic transmission system that single narrow pulse of light becomes dispersed, separated in time, due to effects of chromatic dispersion. Therefore, a chromatic dispersion module""s overall goal is usually to delay wavelengths in a given passband enough to combine all the wavelengths in the passband into a single narrow pulse.
Chromatic dispersion is pulse spreading arising from differences in the speed that light of different wavelengths travel through a material, such as fiber optic cable. Chromatic dispersion is the variation in the propagation speed of light as a function of wavelength. Chromatic dispersion causes a distortion of the optical pulses that propagate through a fiber optic transmission line. As noted, to compensate for the chromatic dispersion in the fiber spans, chromatic dispersion compensating modules (DCMs) are placed periodically in the transmission line. Chromatic dispersion compensating modules add dispersion to the signal, which is ideally equal and opposite in sign, to counteract the dispersion accumulated in the fiber span. The pulses are then reformed to counteract and eliminate the chromatic dispersion-induced distortion within a passband of wavelengths.
In prior technologies, all-pass filters have been tried in dispersion compensation devices. FIG. 1 illustrates a block diagram of a prior art basic etalon-type all-pass filter. The backside mirror has 100% reflector while the front side mirror can have a reflectivity less than 100%. The term all-pass means that no fundamental sources of loss in the device exist, and thus, the theoretical amplitude response equals unity at all wavelengths. The etalon-type all-pass filter therefore only affects the phase of the light. Because dispersion is a change in the phase of the light, this type of filter is well suited to chromatic dispersion compensation.
In FIG. 1, the light travels into the basic etalon-type all-pass filter. A combined input/output fiber sends an optical signal into a collimating lens. The light is collimated and sent at normal incidence into the basic etalon-type all-pass filter. The basic etalon-type all-pass filter produces a variation in the time delay due to the resonate circulation of some wavelengths within the cavity. For wavelengths that are at resonance, the light effectively stays inside the cavity longer than for wavelengths that are off resonance. This causes a wavelength-dependent delay that produces dispersion. Light traveling out of the fiber eventually returns to the collimating lens. However, these basic etalon-type all-pass filter may have unacceptable high amount of insertion loss. Insertion loss is measured by the strength of the optical signal coming out of the all-pass filter as compared to the strength of the optical signal entering the all-pass filter.
Another previous technique of trying to compensate for dispersion compensation was to add a dispersion compensation fiber to a group of optical amplifiers. However, the dispersion compensation fiber was not really adjustable in length or amount of dispersion, and also incurred heavy insertion losses. The heavy insertion losses were some of the non-beneficial effects of adding the dispersion compensation fibers to the system.
Another previous parameter of trying to compensate for dispersion compensation to make the device temperature independent or maintain a temperature so that the variable of temperature does not effect the operation of the device. However, in some cases it might be advantageous to take advantage of temperatures affect on the refractive index of optical components.
Also, most of the above devices are able to tune the dispersion compensation while maintaining a constant dispersion slope. However, in order to compensate for the dispersion of arbitrary fibers, or arbitrary combinations of fibers, a dispersion compensation device that cannot adjust the dispersion and dispersion slope independently may be inadequate.
Various methods, systems, and apparatuses are described. For example, a chromatic dispersion compensation module includes an input fiber, an output fiber, a lens, and an etalon resonator. The input fiber has a first core with a center. The output fiber has a second core with a center. The input fiber is adjacent to the output fiber. The spacing between the center of the first core and the center of the second core is affixed to less than one hundred and twenty microns. The input fiber routes an optical signal to a lens. The lens routes the optical signal to the etalon resonator. The etalon resonator has reflectors with fixed reflectivity and a variable optical length to induce a wavelength-dependent delay into the optical signal. The etalon resonator routes the optical signal to the output fiber through the lens.
A chromatic dispersion compensation module that includes an input fiber, an output fiber, a lens, and a first etalon is described. The input fiber is adjacent to the output fiber. The input fiber routes an optical signal to the lens. The first etalon resonator has reflectors with wavelength-dependent reflectivity and a variable optical length to induce a wavelength-dependent delay into the optical signal. The lens routes the optical signal to the first etalon resonator. The first etalon resonator routes the optical signal to the output fiber through the lens. A temperature control device affixes to the first etalon resonator. The temperature control device controls temperature of the first etalon resonator independent of any other etalon resonator.
A method of inducing wavelength-dependent delays in to an optical signal is described. The optical signal is collimated. The optical signal is routed to an etalon resonator. The resonant center wavelength of the etalon resonator is tuned by adjusting optical path length of the etalon resonator. The optical signal is routed, via a non-normal angle of incidence, to an output. The non-normal angle of incidence incurs a figure of merit of less than two tenths.
A tunable chromatic dispersion compensation module that includes a first set of etalons and a second set of etalons is described. The first set of etalons has a Free Spectral Range equal to a first value. The first set of etalons creates a dispersion over a first passband of optical wavelengths. The second set of etalons has a Free Spectral Range offset from the first value and a second passband of optical wavelengths that is greater in bandwidth than the band of optical wavelengths in the first passband. The second set of etalons creates a dispersion over the second passband of optical wavelengths. The dispersion over the first passband of optical wavelengths and the dispersion over the second passband of optical wavelengths sums together.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.